Regenerated Power Accumulator for Rod Lift Drive

ABSTRACT

A variable speed drive for use in connection with a beam pumping unit has a pair of DC bus rails that include a positive DC bus rail and a negative DC bus rail. The variable speed drive further comprises a rectifier section, an inverter section connected between the DC bus rails and a primary capacitor bank. The primary capacitor bank includes a primary capacitor and an overvoltage switch. The variable speed drive further includes an auxiliary capacitor bank that has an auxiliary capacitor, a charge diode connected between the auxiliary capacitor and the overvoltage switch, and a discharge diode connected between the auxiliary capacitor and the positive DC bus rail.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/836,602 filed Apr. 19, 2019 entitled “RegeneratedPower Accumulator for Rod Lift Drive,” the disclosure of which is hereinincorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to oilfield equipment, and inparticular to a drive system for surface-mounted reciprocating-beam,rod-lift pumping units.

BACKGROUND

Hydrocarbons are often produced from wells with reciprocating downholepumps that are driven from the surface by pumping units. A pumping unitis connected to its downhole pump by a rod string. Although severaltypes of pumping units for reciprocating rod strings are known in theart, walking beam style pumps enjoy predominant use due to theirsimplicity and low maintenance requirements.

Beam pumping units and their upstream drive components are exposed to awide range of loading conditions. These vary by well application, thetype and proportions of the pumping unit's linkage mechanism andcounterbalance matching. The primary function of the pumping unit is toconvert rotating motion from the prime mover (engine or electric motor)into reciprocating motion above the wellhead. This motion is in turnused to drive a reciprocating down-hole pump via connection through asucker rod string.

The “4-bar linkage” comprising the articulating beam, pitman, cranks,and connecting bearings processes the load from the polished rod intoone component of the gear box torque (well torque). The other component,counterbalance torque, is adjusted on the pumping unit to yield thelowest net torque on the gearbox. Counterbalance torque can be adjustedin magnitude but typically not in phase (timing) with respect to thewell load torque.

Counterbalance may be provided in a number of forms ranging frombeam-mounted counterweights, to crank-mounted counterweights (as shownin FIG. 1), to compressed gas springs mounted between the walking beamand base structure to name only a few. The primary goal in incorporatingcounterbalance is to offset a portion of the well load approximatelyequal to the average of the peak and minimum polished rod loadsencountered in the pumping cycle. This technique typically minimizes thetorque and forces at work on upstream driveline components reducingtheir load capacity requirements and maximizing energy efficiency.

Although generally effective at offsetting a portion of the loadproduced by the downhole components of the reciprocating pumping system,the rotating mass of the crank-mounted counterweights are difficult torapidly adjust under advanced control schemes. The elasticity of thesucker rod string may present an oscillatory response when exposed tovariable loads. The motion profile of the driving pumping unit combinedwith the step function loading of the pump generally leaves little timefor the oscillations to decay before the next perturbation isencountered. The flywheel effect produced by massive rotating componentswithin the pumping unit resists rapid changes in speed. Attempts tosubstantially alter speed within the pumping cycle have generallyconsumed disproportionately more power which negatively affectsoperating cost.

In many cases, the beam pumping unit is driven by an electric motor(prime mover) that is controlled by a variable speed drive. The electricmotor is connected to the gearbox to rotate the crankshaft armsaccording to a control scheme. During portions of the reciprocatingcycle, the motor is energized by the drive to apply torque to thecrankshaft through the gearbox. In other portions of the cycle, theweight of the rod string, counterbalance weights, and other portions ofthe beam pumping unit cause the electric motor to passively rotatewithout the application of drive current. During this portion of thepumping cycle, the electric motor acts as a generator and produces acurrent that may be fed back into the variable speed drive.

To accommodate the generated current, the variable speed drive mayinclude a dynamic braking resistor (DBR) that is configured to releaseexcess energy as heat. Prior art variable speed drives often include arectifier, a capacitor bank, a dynamic braking section, and an invertermotor output section connected along common bus rails for connecting athree phase power input to a three phase output provided to the motor.

When the motor is drawing power, diodes in the charging section chargethe capacitor bank and an insulated-gate bipolar transistor (IGBT)bridge arrangement in the inverter motor output section modulatescapacitor voltage to control current in the motor windings. When themotor is regenerating power, due to braking action, as the rod stringpulls the walking beam downward, for example, diodes in the invertermotor output section transfer power to the capacitor bank, causing thecapacitor bank voltage to rise. If the capacitor bank voltage exceeds athreshold amount, a dynamic braking resistor can be temporarily switchedon to rheostatically dissipate the energy from the variable speed drive.

Although generally accepted, the reliance on dynamic braking resistorsto control overvoltage situations can be inefficient as a large portionof the regenerated energy is intentionally dissipated as heat. There is,therefore, a need for an improved variable speed drive that moreefficiently manages electrical current generated by the motor of a beampumping unit.

SUMMARY OF THE INVENTION

In one aspect, embodiments of the present invention a variable speeddrive for use in connection with a beam pumping unit has a pair of DCbus rails that include a positive DC bus rail and a negative DC busrail. The variable speed drive further comprises a rectifier section, aninverter section connected between the DC bus rails and a primarycapacitor bank. The primary capacitor bank includes a primary capacitorand an overvoltage switch. The variable speed drive further includes anauxiliary capacitor bank that has an auxiliary capacitor, a charge diodeconnected between the auxiliary capacitor and the overvoltage switch,and a discharge diode connected between the auxiliary capacitor and thepositive DC bus rail.

In another aspect, embodiments of the present invention include a motordrive assembly for controlling the operation of a plurality of beampumping units. In these embodiments, the motor drive assembly includes acentral auxiliary capacitor bank and a plurality of variable speeddrives. Each of the plurality of variable speed drives is connectedbetween the central auxiliary capacitor bank and a corresponding one ofthe plurality of beam pumping units.

In yet another aspect, embodiments of the present invention include anenergy efficient drive system for a beam pumping unit. The energyefficient drive system includes a variable speed drive, an externalswitch connected to the DC bus rails of the variable speed drive, and anauxiliary capacitor bank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a beam pumping unit and well.

FIG. 2 is a circuit diagram for a variable speed drive constructed inaccordance with an exemplary embodiment.

FIG. 3 is a depiction of multiple beam pumping units connected to acommon auxiliary capacitor bank.

WRITTEN DESCRIPTION

FIG. 1 shows a beam pumping unit 100 constructed in accordance with anexemplary embodiment of the present invention. The beam pumping unit 100is driven by a prime mover 102, typically an electric motor or internalcombustion engine. The rotational power output from the prime mover 102is transmitted by a drive belt 104 to a gearbox 106. The gearbox 106provides low-speed, high-torque rotation of a crankshaft 108. Each endof the crankshaft 108 (only one is visible in FIG. 1) carries a crankarm 110 and a counterbalance weight 112. The reducer gearbox 106 sitsatop a sub-base or pedestal 114, which provides clearance for the crankarms 110 and counterbalance weights 112 to rotate. The gearbox pedestal114 is mounted atop a base 116. The base 116 also supports a Samson post118. The top of the Samson post 118 acts as a fulcrum that pivotallysupports a walking beam 120 via a center bearing assembly 122.

Each crank arm 110 is pivotally connected to a pitman arm 124 by a crankpin bearing assembly 126. The two pitman arms 124 are connected to anequalizer bar 128, and the equalizer bar 128 is pivotally connected tothe rear end of the walking beam 120 by an equalizer bearing assembly130, commonly referred to as a tail bearing assembly. A horse head 132with an arcuate forward face 134 is mounted to the forward end of thewalking beam 120. The face 134 of the horse head 132 interfaces with aflexible wire rope bridle 136. At its lower end, the bridle 136terminates with a carrier bar 138, upon which a polish rod 140 issuspended.

The polish rod 140 extends through a packing gland or stuffing box 142on a wellhead 144 above a well 200. A rod string 146 of sucker rodshangs from the polish rod 140 within a tubing string 148 located withinthe well casing 150. The rod string 146 is connected to a subsurfacepump 202. In a reciprocating cycle of the beam pumping unit 100, wellfluids are lifted within the tubing string 148 during the rod string 146upstroke. In accordance with well-established rod lift pump design, astationary standing valve and reciprocating traveling valve cooperate tolift fluids to the surface through the tubing string 148.

The motor 102 is driven by a variable speed drive 152. The variablespeed drive 152 receives a source of electrical power from a powersource such as an established electrical grid or a generator. The inputcurrent to the variable speed drive 152 may be processed through aninput transformer to adjust the voltage of the current. Turning to FIG.2, shown therein is a simplified depiction of the major circuits within,or connected to, the variable speed drive 152. In the exemplaryembodiment depicted in FIG. 2, the variable speed drive 152 includes arectifier (converter) section 154, an inverter section 156, a primarycapacitor bank 158 and an auxiliary capacitor bank 160. Each of thesections of the variable speed drive 152 is interconnected by common DCbus rails (+, −) 162, 164.

In accordance with well-established motor drive technology, therectifier section 154 converts the input voltage to the variable speeddrive 152 to direct current coupled to the DC bus rails 162, 164. Theinverter section 156 is utilized to convert the DC bus voltage to avariable frequency AC signal, in response to motor drive controlcommands in the variable speed drive 152. In some configurations, theoutput from the inverter section 156 is provided directly to the motor102 or to an intermediate step-up transformer (not shown). The invertersection 156 of the variable speed drive 152 can be configured to producea six-step commutation sequence that can be adjusted manually orautomatically to adjust the operating parameters of the beam pumpingunit 100. The inverter section 156 may include a plurality ofinsulated-gate bipolar transistors (IGBTs) arranged in a bridgeconfiguration to selectively adjust the output of the variable speeddrive 152.

The primary capacitor bank 158 includes a primary capacitor 166 and anovervoltage switch 168. The primary capacitor 166 is charged by the DCbus rails 162, 164. During a driving mode of operation, the voltagewithin the primary capacitor 166 is drained as current is applied to themotor 102 by the variable speed drive 152. During a braking mode ofoperation, the motor 102 acts as a generator to produce current and acorresponding retarding torque. The current generated by the motor 102is fed back to the variable speed drive 152 and charges the primarycapacitor 166. Although a single primary capacitor 166 is depictedwithin FIG. 2, it will be appreciated that the primary capacitor bank158 may include a plurality of primary capacitors 166 linked together toprovide an aggregated capacitance.

In exemplary embodiments, the overvoltage switch 168 is aninsulated-gate bipolar transistor (IGBT). The overvoltage switch 168 canbe switched on to connect the auxiliary capacitor bank 160 to theprimary capacitor bank 158 through the DC bus rails 162, 164. Theauxiliary capacitor bank 160 generally provides the variable speed drive152 with sufficient capacity to minimize the risk of the voltage acrossthe DC bus rails 162, 164 exceeding established thresholds. Unlike priorart systems in which an overvoltage switch would be used to route excessvoltage for dissipation on a dynamic braking resistor (DBR), theovervoltage switch 168 of the variable speed drive 152 is configured toroute excess voltage generated during a braking mode of operation to theauxiliary capacitor bank 160 for storage and subsequent use during adriving mode of operation.

The auxiliary capacitor bank 160 includes an auxiliary capacitor 170, acharge diode 172 and a discharge diode 174. Although a single auxiliarycapacitor 170 is depicted, it will be appreciated that the auxiliarycapacitor bank 160 may include a plurality of auxiliary capacitors 170linked together to provide an aggregated capacitance. In exemplaryembodiments, the combined capacitance of the primary capacitor 166 andthe auxiliary capacitor 170 is designed to accommodate more than themaximum anticipated charge produced by the motor 102 during theregenerating (braking) phase of operation.

The charge diode 172 permits the unidirectional flow of current from theDC bus rail 162 to the anode side of the auxiliary capacitor 170. Thedischarge diode 174 permits the unidirectional flow of current from theanode side of the auxiliary capacitor 170 to the DC bus rail 162. Inthis way, the charge diode 172 and discharge diode 174 cooperate toautomatically control the charge at the auxiliary capacitor 170 based ondifferences in voltage between the DC bus rail 162 and the anode side ofthe auxiliary capacitor 170. The auxiliary capacitor bank 160 optionallyincludes a resistor 176 between the charge diode 172 and the auxiliarycapacitor 170. The resistor 176 can be configured to reduce the voltageat the anode side of the auxiliary capacitor 170 so that it remains lessthan the voltage on the DC bus rail 162.

During a braking mode of operation, current generated by the motor 102is fed to the primary capacitor bank 158. As the voltage in the primarycapacitor 166 begins to rise toward a predetermined limit, theovervoltage switch 168 is automatically closed to connect the auxiliarycapacitor bank 160. If the voltage on the DC bus rail 162 is greaterthan the voltage at the anode side of the charge diode 172, currentflows through the charge diode 172 to the anode side of the auxiliarycapacitor 170. The voltage at the auxiliary capacitor 170 continues torise during the regeneration (braking) phase of the pumping cycle, butcurrent does not flow out of the auxiliary capacitor 170 until thevoltage on the DC bus rail 162 is less than the voltage at the anodeside of the auxiliary capacitor 170.

When the variable speed drive 152 switches to a driving mode ofoperation, the primary capacitor 166 begins to drain as current is fedthrough the inverter section 156 to the motor 102. As the voltage on theprimary capacitor 166 and the DC bus rail 162 drops, current flows outof the auxiliary capacitor 170 through the discharge diode 174 to the DCbus rail. In this way, the charge in the auxiliary capacitor 170 isreleased during the driving mode of operation to offset a portion of thepower that would otherwise be required to operate the motor 102. It willbe appreciated that in well balanced pumping systems, the chargegenerated by the motor 102 during the downstroke portion of the pumpingcycle may not exceed the limits of the primary capacitor 166. In thatsituation, the overvoltage switch 168 is not activated and the auxiliarycapacitor bank 160 remains functionally disconnected from the primarycapacitor bank 158.

The auxiliary capacitor bank 160 can be easily retrofitted onto existingvariable speed drives, which often include overvoltage switches thatdirect current to a conventional dynamic braking resistor. In this way,the conventional dynamic braking resistor can be easily replaced withthe more efficient auxiliary capacitor bank 160. The auxiliary capacitorbank 160 can be housed inside the variable speed drive 152 or in anappropriate cabinet that is connected to the variable speed drive 152.In other situations, an external switch connected to the DC bus rails162, 164 can be easily installed to provide a mechanism for connectingthe auxiliary capacitor bank 160 to existing variable speed drives 152that do not include overvoltage switching mechanisms and dynamic brakingresistors. In certain applications, it may be desirable to implement theauxiliary capacitor bank 160 in combination with a traditional dynamicbraking resistor.

As depicted in FIG. 3, in some applications a common central auxiliarycapacitor bank 160 is connected to multiple variable speed drives 152that are each connected to a motor 102 of a different beam pumping unit100. In this embodiment, the common auxiliary capacitor bank 160 must beappropriately sized to accommodate the aggregate charge anticipated bythe simultaneous regeneration of power from the multiple beam pumpingunits 100. In this embodiment, the charge and discharge of the commonauxiliary capacitor bank 160 remains an automatic function as the chargeand discharge diodes 172, 174 control the flow of current to theauxiliary capacitor 170 without intelligent intervention.

It is to be understood that even though numerous characteristics andadvantages of various embodiments of the present invention have been setforth in the foregoing description, together with details of thestructure and functions of various embodiments of the invention, thisdisclosure is illustrative only, and changes may be made in detail,especially in matters of structure and arrangement of parts within theprinciples of the present invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed. It will be appreciated by those skilled in the art that theteachings of the present invention can be applied to other systemswithout departing from the scope and spirit of the present invention.

What is claimed is:
 1. A variable speed drive for use in connection witha beam pumping unit, the variable speed drive comprising: a pair of DCbus rails, wherein the pair of DC bus rails comprise a positive DC busrail and a negative DC bus rail; a rectifier section connected betweenthe DC bus rails; an inverter section connected between the DC busrails; a primary capacitor bank connected between the DC bus rails,wherein the primary capacitor bank comprises: a primary capacitor; andan overvoltage switch; and an auxiliary capacitor bank, wherein theauxiliary capacitor bank comprises: an auxiliary capacitor; a chargediode connected between the auxiliary capacitor and the overvoltageswitch; and a discharge diode connected between the auxiliary capacitorand the positive DC bus rail.
 2. The variable speed drive of claim 1,wherein the overvoltage switch is selected from the group consisting ofinsulated-gate bipolar transistors, semiconductor switches andmechanical switches.
 3. The variable speed drive of claim 2, wherein theovervoltage switch is an insulated-gate bipolar transistor.
 4. Thevariable speed drive of claim 2, wherein the auxiliary capacitor bankfurther comprises a resistor between the charge diode and the auxiliarycapacitor.
 5. A motor drive assembly for controlling the operation of aplurality of beam pumping units, the motor drive assembly comprising: acentral auxiliary capacitor bank; and a plurality of variable speeddrives, wherein each of the plurality of variable speed drives isconnected between the central auxiliary capacitor bank and acorresponding one of the plurality of beam pumping units.
 6. The motordrive assembly of claim 5, wherein each of the plurality of variablespeed drives is connected to the central auxiliary capacitor bankthrough an overvoltage switch that permits the central auxiliarycapacitor bank to be automatically charged by the corresponding variablespeed drive.
 7. The motor drive assembly of claim 6, wherein each of theovervoltage switches is selected from the group consisting ofinsulated-gate bipolar transistors, semiconductor switches andmechanical switches.
 8. The motor drive assembly of claim 7, whereineach of the overvoltage switches is an insulated-gate bipolartransistor.
 9. The motor drive assembly of claim 5, wherein each of theplurality of variable speed drives comprises: a pair of DC bus rails,wherein the pair of DC bus rails comprise a positive DC bus rail and anegative DC bus rail; a rectifier section connected between the DC busrails; an inverter section connected between the DC bus rails; and aprimary capacitor bank connected between the DC bus rails, wherein theprimary capacitor bank comprises a primary capacitor.
 10. The motordrive assembly of claim 5, wherein the central auxiliary capacitor bankcomprises: an auxiliary capacitor; a charge diode connected between theauxiliary capacitor and the overvoltage switches of each of theplurality of variable speed drives; and a discharge diode connectedbetween the auxiliary capacitor and the positive DC bus rails of each ofthe plurality of variable speed drives.
 11. The motor drive assembly ofclaim 10, wherein the auxiliary capacitor bank further comprises aresistor between the charge diode and the auxiliary capacitor.
 12. Anenergy efficient drive system for a beam pumping unit, the energyefficient drive system comprising: a variable speed drive, an externalswitch connected to the DC bus rails of the variable speed drive; and anauxiliary capacitor bank.
 13. The energy efficient drive system of claim12, wherein the variable speed drive comprises: a pair of DC bus rails,wherein the pair of DC bus rails comprise a positive DC bus rail and anegative DC bus rail; a rectifier section connected between the DC busrails; and an inverter section connected between the DC bus rails. 14.The energy efficient drive system of claim 13, wherein the variablespeed drive further comprises a primary capacitor bank connected betweenthe DC bus rails.
 15. The energy efficient drive system of claim 14,wherein the primary capacitor bank further comprises an overvoltageswitch.
 16. The energy efficient drive system of claim 15, wherein theovervoltage switch is selected from the group consisting ofinsulated-gate bipolar transistors, semiconductor switches andmechanical switches.
 17. The energy efficient drive system of claim 16,wherein the overvoltage switches is an insulated-gate bipolartransistor.
 18. The energy efficient drive system of claim 12, whereinthe auxiliary capacitor bank comprises: an auxiliary capacitor; a chargediode connected between the auxiliary capacitor and the external switch;and a discharge diode connected between the auxiliary capacitor and thepositive DC bus rail.
 19. The energy efficient drive system of claim 18,wherein the auxiliary capacitor bank further comprises a resistorbetween the charge diode and the auxiliary capacitor.